Cruz has a chance to TKO Trump and knock out the Bush-Kasich-Christie establishment candidates by focusing his attacks on all of them, making this a race where the party is guaranteed one of two real conservatives as the nominee, himself or Rubio.
Or, Cruz can join the establishment in its all-out assault on Rubio. This will revive Trump's campaign by keeping Trump's numbers up and giving him a big win in New Hampshire. It will also bring one of those establishment governors as a fourth viable candidate in South Carolina and beyond.
One choice is good for the party and the country, and if Cruz believes in his organization it still gives him a chance to win as the front runner and Trump killer.
The other choice risks ruin for the party by taking his foot off the throat of the Trump and establishment campaigns and making it possible for one or two nonconservatives to win the nomination.
What's the right thing for Cruz to do?

If you think the right thing for the country is for YOU to be President, you have to play hardball against your strongest perceived threat. So I raise this point not to knock Cruz for being somehow immoral if he attacks Rubio. That's fair game in politics. I raise it only to show that Cruz' behavior belies his claim to be always working from pristine principle (as opposed to everyone else) and because my "injustice" and "false" hackles go up when I see Rubio dismissed as the "establishment" candidate, when in fact he was a Tea Party candidate, not supposed to have won, and he has been a rock-solid conservative, more effective in actually moving the ball forward than Ted Cruz. Here are two links you can follow to other links to amply demonstrate this point.

Here's an Atlantic"expose" of Rubio's "feigned" moderation that makes the point anyone interested in liberty and small government ought to be paying attention to: not who can say the loudest, boldest conservative things, but who can persuade voters and move the ball down the field?

Rubio has mastered the same technique Barack Obama used so effectively when he was seeking the presidency. When faced with a controversial issue, he doffs his cap to the other side, pleads for civility and respect, insists that it’s a hard call—and then comes out exactly where you’d expect him to come out. On social issues, Rubio is as predictably conservative as Obama is predictably liberal. What they share is their moderate-sounding rhetorical style.

The real establishment candidate, Jeb Bush, is spending all his time and money attacking Rubio viciously (including, I believe, though cannot prove, being the source of the stupid NYT piece running the shocking, scandalous news that Rubio's wife has had parking tickets and that Rubio once bought a small boat. In Florida). Bush's campaign surrogate, Lindsay Graham, has just attacked Rubio for being too pro-life. (C'mon, Jeb. PLEASE get out before you beclown yourself any further). That is what the "establishment" does, and it has nothing to do with Rubio.

And as for who is likely to have a better understanding of the difficulties of normal Americans, is that going to be the Princeton & Harvard-educated, SCOTUS-clerking Ted Cruz, whose wife works for Goldman-Sachs? Or the football-scholarship/ community college/ finally got his act together at U. Florida, and then JD from University of Miami Rubio, whose wife was once a Dolphins cheerleader and is now ashamed of it? The Rubios are not "establishment" -- they're normal middle-class Americans. He just happens to come from Cuban immigrants, so he knows in his bones what is great (or at least could be great again) about our country, and he's enormously principled and scrappy in doing something about it. He's an outspoken pro-lifer, outspoken defender of marriage, outspoken defender of persecuted Christians (whereas Cruz -- my big bugaboo against him-- threw them under the bus to fundraise).

By all means, let Cruz & Rubio duke it out and show us which of them would be a better conservative President. But the scorn for Rubio as a Jeb-like establishment figure is either taking Cruz' word for it, prejudice against his good looks, or a matter of not being truly familiar with Rubio's record. Here, let Rush Limbaugh tell it.